DESCRIPTION: (Investigator's abstract) In utero hematopoietic stem cell transplantation (IUHSCTx) is a promising therapeutic approach for the treatment of a variety of congenital hematologic disorders. However, in the absence of a selective advantage for normal cells, clinical and experimental efforts at IUHSCTx have resulted in limited or no detectable engraftment, supporting the existence of major barriers to engraftment in the hematopoietically competitive recipient. The long-term objective of this project is to develop clinically applicable strategies to increase engraftment to therapeutic levels. Overcoming the engraftment barrier would allow increased clinical application of IUHSCTx with the potential for reduced morbidity, mortality, and cost, relative to currently available therapy. The specific aims of this proposal are: 1. To optimize donor cell subpopulations for engraftment after in utero HSC transplantation. The requirements for homing, lodgement and engraftment to the fetal liver are poorly understood. Donor cell populations differ in their engraftment efficiency. Using novel strategies, specific donor cell populations will be compared for their early kinetics of engraftment, the relative repopulating ability of engrafted cells, and the competitive capacity after IUHSCTx of equivalent repopulating populations. This will provide insight into the relative ability of specific donor populations to engraft after IUHSCTx and into the relative importance of initial number of HSC engrafted, versus the competitive capacity of the donor population, for the achievement of high level engrafment. 2. To investigate the efficacy of immune based strategies to enhance engraftment prenatally, and to increase levels of chimerism postnatally. This Specific Aim has three goals: 1) To identify and validate specific donor derived cell populations that can facilitate prenatal engraftment of enriched HSC without graft versus host disease (GVHD); 2) to investigate prenatal immunotherapeutic strategies to suppress GVHD and improve engraftment of non-enriched donor populations after IUHSCTx; and 3) to test the efficacy of immunotherapeutic approaches after birth to reduce or eliminate host hematopoiesis in the mixed chimeric recipient made tolerant by IUHSCTx. The studies in this proposal are designed to identify and test strategies to increase engraftment after IUHSCTx. This would allow application of this approach to a broad range of prenatally diagnosed genetic disorders.